In a simple ring laser gyroscope (gyro or RLG) two beams of substantially monochromatic light travel in opposite directions in a closed optical path. Rotation within the plane of the closed optical path causes the effective path length traveled by one beam to increase while the path length traveled by the other beam decreases. Since the frequency of oscillation of the beams of light used in the gyroscope depends upon the effective length of the closed optical path, the changes in optical path length produce an increased frequency in one of the beams and a decreased frequency in the other. The amount of frequency change is indicative Of the rotation rate of the closed optical path relative to a plane in which rotation is to be sensed. It can be calculated by means of the interference pattern created by interaction of the two light beams.
As the rotation rate relative to the closed optical path approaches zero, however, the difference in frequency between the two beams becomes small, and the two beams tend to resonate together or "lock-in" so that the two beams oscillate at only one frequency. The rotation rate below which lock-in occurs is commonly referred to as the "lock-in rate." When the gyroscope is rotating at rotation rates below the lock-in rate and the beams are locked in, it becomes impossible to accurately measure rotation. This inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in navigational systems. Thus, much developmental work has been conducted in the field of ring laser gyroscopes for the purpose of reducing or eliminating the effects of lock-in.
It is well known in the art, for instance, to reduce the effects of lock-in by introducing dither. A sinusoidal frequency bias can be mechanically or electro-optically introduced in at least one of the two oppositely travelling beams of light. To be effective, the frequency bias introduced must be greater than the frequency difference which occurs just prior to lock-in for a majority of time. In such systems, the sign or polarity of the frequency bias introduced is periodically reversed so that after one complete cycle of the periodically reversing bias, the time integrated frequency difference between the two beams of light is substantially zero.
The sinusoidal frequency biasing approach reduces but does not eliminate lock-in error. For instance, each time the polarity of the frequency bias reverses, the frequency bias becomes zero and the two light beams can lock-in. This type of lock-in is termed "dynamic lock-in." Although the time intervals in which the beams could dynamically lock-in are very short and, consequently, any possibly resulting gyroscopic error is greatly reduced, nevertheless the resulting error accumulates in the gyroscopic output angle signal. In time the error can amount to a bothersome level, particularly in precision navigational systems.
An improved biasing system was disclosed in U.S. Pat. No. 3,467,472 issued to Joseph E. Killpatrick, and assigned to the present assignee. U.S. Pat. No. 3,467,472 discloses a method of reducing cumulative error due to dynamic lock-in. In this method, a randomizing bias is added to the frequency bias. The randomizing bias causes the times at which the overall dithering signal reverses to shift randomly. This randomization results in a reduced average cumulative error. The discussion in U.S. Pat. No. 3,467,472 of ring laser gyroscopes, the lock-in problem and possible solutions is hereby incorporated by reference.
Typically, the randomization signal is noise that has been passed through a low pass filter. The resulting low pass filtered noise is summed with the normal dither sinusoidal signal and injected into the dither drive mechanism. Turnaround angles are randomized, but, since much of the injected electrical noise power is not effective in changing the turnaround angle, this process is not near optimum in terms of efficiency.
A second approach to an improved biasing system was disclosed in U.S. Pat. No. 4,695,160, issued to Werner H. Egli, and also assigned to the present assignee. U.S. Pat. No. 4,695,160 teaches that the zero rate crossings in each dither cycle constitute a source of dynamic, lock-in error and that the error is cumulative. That cumulative error can, however, be substantially reduced by altering the instantaneous phase difference between the two counter-travelling beams of light at successive zero rate crossings by a predetermined value. The discussion in U.S. Pat. No. 4,695,160 of ring laser gyroscopes, the lock-in problem and possible solutions is hereby incorporated by reference.
Egli teaches that the instantaneous phase difference can be manipulated by altering the maximum positive and negative dither angle amplitudes by a preselected amount and in phase with the zero rate crossings. He suggests summing two in phase sinusoids (one at the dither frequency and the second at an integer fraction of the dither frequency) in order to create a composite sinusoidal dither signal in which the maximum positive and negative amplitudes are periodically altered. This causes a corresponding change in the maximum positive and negative dither angles of the gyro. As long as the change is greater than half of an interference fringe spacing (or "count"), the incremental lock-in error for two successive dither cycles will be approximately zero. To ensure this, Egli, in one embodiment, suggests setting the amplitude of the second sinusoidal signal to a level necessary to cause a maximum dither angle amplitude of .sqroot.2/2 when operating independently of the first sinusoidal signal. In addition, Egli suggests that further benefits can be achieved by modifying the first sinusoidal signal to have a maximum amplitude which changes to a new, random level every other dither cycle.
The approach taught by Egli requires the generation of two in-phase sinusoidal components, one at an integer fraction of the frequency of the other. It requires photodetectors to measure the phase angle between the two light beams in order to detect the zero rate crossing and significant processing power in order to adjust the dither based on the phase angle measurements.
It is apparent that there is a need for an improved method of reducing lock-in error in a ring laser gyro. The addition of noise to the dither of a ring laser gyro has proven effective at reducing such error. What is needed, however, is a method of optimally injecting noise to a dither drive signal in order to randomize the cycle to cycle turnaround angles in a way which most efficiently produces angular changes between cycles. It is preferable that such a method minimize power dissipation while, at the same time, lowering the high frequency current required from the RLG power supply.